Because the manufacture of complementary double poly bipolar transistors customarily requires the use of two separate emitter doping masks and two high dose emitter implants, in addition to two extrinsic base doping masks and two high dose extrinsic base implants, its cost and cycle time is undesirably high.
One proposal to reduce the complexity of such a mask set is described in the U.S. patent to Ikeda, U.S. Pat. No. 5,175,607, and involves only two implants to dope the base and emitter poly of both transistor polarity types. A sectional view of the architecture at the emitter poly formation stage of the Ikeda process (corresponding to FIG. 3L of the patent) is diagrammatically illustrated in FIG. 1. As shown therein, the Ikeda NPN device 1 has a conventional double poly transistor structure, in which the emitter poly layer 13 extends laterally above the base contact poly layer 11. This extension of the emitter poly layer allows the top of the emitter poly 13 to be wider than the width of the gap in the base contact poly 11, through which it contacts the base, and which defines the width of the emitter junction (desired to be small for best high frequency performance). The top width can be made as large as necessary to accommodate the emitter contact (where contact metal 19 meets emitter poly layer 13), including allowance for alignment tolerance, no matter how narrow the aperture in the base poly is made to improve high frequency performance.
The PNP device in FIG. 1 does not have the conventional double poly structure. In the PNP device, the emitter poly layer 15 is formed on the surface of the base, prior to formation of the base contact poly layer 17. The width of the emitter is set by the width of the emitter poly stripe. This stripe is formed with vertical side walls, to allow formation of side wall spacers 16, which separate the emitter poly layer 15 and the base contact poly layer 17. The base poly layer 17 is deposited and patterned after the spacers have been formed.
As a result of this structure, the emitter contact is necessarily narrower than the emitter junction (whose width is defined by the width of the contact between the emitter poly layer 15 and the island, because the emitter dopant diffuses from the emitter poly layer 15 into the base after the emitter poly has been patterned), if it is formed directly above the emitter, as shown in FIG. 1. A wider than desired emitter may be required to accommodate the smallest emitter contact which can be made in this case.
As diagrammatically illustrated in FIG. 1A, a wider contact, shown at 14, can be used if it is placed on a `remote` portion of the emitter poly layer 15 not in contact with the base. A disadvantage of such a remotely located contact is the fact that emitter current must flow laterally through the emitter poly layer 15 from the emitter contact 14 to the emitter, which results in high emitter resistance, which reduces performance.
Further, in a conventional double poly transistor process, the base contact poly (base poly for short) and the extrinsic base, which is doped from the base poly by out-diffusion, are doped before the emitter is formed. This presence of extrinsic base doping during emitter poly formation limits the amount of high temperature treatment that can be performed in the emitter formation and doping process, since the extrinsic base diffuses vertically and laterally into the collector during the emitter processing. This limitation is particularly severe for shallow base, narrow emitter structures, such as are used for high frequency devices.